1. Field of the Invention
The present invention relates generally to a semiconductor memory device having a circuit for changing electrical characteristics, and more particularly to a semiconductor memory device permitting the normality/abnormality of fuse blowing therein to be checked in the molded state.
2. Description of the Background Art
There have been semiconductor memory devices which permit adjustment of internal power supply potentials or replacement with redundant memory cells by blowing fuses.
An example of the configuration of such a conventional semiconductor memory device having a replaceable redundant circuit will be described.
A conventional semiconductor memory device 2000 shown in FIG. 20 includes a memory block #1, a redundant memory array #11, a main decode circuit 2, a redundant decode circuit 3, and a redundant comparator circuit 4.
Memory block #1 includes a plurality of memory cells (not shown) arranged at crossings of word lines and bit lines.
Main decode circuit 2 which selects a memory cell in memory block #1 has a column decode circuit and a row decode circuit.
Redundant memory array #11 has a plurality of redundant memory cells which are not shown. Redundant decode circuit 3 which selects a memory cell in redundant memory array #11 has a column decode circuit and a row decode circuit.
Redundant comparator circuit 4 controls main decode circuit 2 and redundant decode circuit 3 in order to replace a memory cell with a defect (hereinafter referred to as "defective cell") with a redundant memory cell.
In semiconductor memory device 2000, a defective cell found in memory cell block #1 during processing wafer is replaced with a redundant cell in the following procedure.
The address of the defective cell (hereinafter referred to as "defective address") is searched, and checked for the possibility of replacement with a redundant memory cell. If the defective address is replaceable, a polysilicon fuse portion corresponding to the defective address in redundant comparator circuit 4 is blown.
After this operation and the step of forming a protection film, only the products which have passed the final selection test are marketed.
Thus, if a defective cell is selected, redundant comparator circuit 4 controls main decode circuit 2 and redundant decode circuit 3 so that a replaced redundant memory cell is selected.
After the manufacture, in the process of analyzing or evaluating defects in semiconductor memory device 2000, it is critical to know if fuses are blown normally or abnormally. In the conventional semiconductor memory device 2000, the package should be opened for checking the normality/abnormality of the fuse blow using an electron microscope.
Furthermore, since it is not possible to determine if fuses are blown normally in the molded state, products with abnormally blown fuses may pass the final selection test to be marketed.